Aqueous lithium ion batteries are currently being developed and present several advantages over conventional alkaline, nickel metal hydride (NiMH) or lead acid batteries [1]. The pH of the electrolyte used in aqueous lithium batteries is close to 7, which makes these systems inherently safer.
Though non aqueous lithium ion batteries have a better energy density because of the higher voltage available in organic electrolytes, they are hazardous, expensive and contain highly toxic chemicals in their current state. In comparison, an aqueous lithium battery is safe, cheap and environmentally friendly.
Aqueous mixed lithium/proton batteries offer the perspective of storing more energy than proton or lithium batteries do independently [2].
Synthetic or natural vanadium(IV) oxide compounds are of interest in fundamental research [3] and for industrial applications [4] because of their structural peculiarities and their extended redox properties.
H2V3O8 is a known compound that has already been described in 1970 [5] and that was structurally analyzed in 1990 [6]. Its composition and structure has been modified in several ways in order to improve its use as electrode material in batteries. However, only the compounds obtained by oxidation to an all vanadium(V) oxohydroxide in which part of the hydrogen has been substituted by alcaline metal cations have been described so far [7]. So far practically no compounds obtained by reduction of H2V3O8, in which the oxidation states of the vanadium are mixed 4+/5+, only 4+ or 4+/3+, have been characterized. One exception is [8], in which LixH2V3O8 phases are described.
A V(IV) compound is known in the form of the mineral doloresite [9] that is a hydrated form of VO2. Doloresite is of monoclinic symmetry with a=19.64±0.06, b=2.99±0.01, c=4.89±0.02 and β=103°55′±5′ [9]. Doloresite can be formulated as V3O4(OH)4 or H4V3O8.
Hence, it is a general object of the invention to provide a compound that can accommodate reversibly both lithium ions and protons.